Friction materials for use in brake systems for various types of vehicles, industrial machines, and so on are required to have high and stable coefficients of friction and have excellent wear resistance. To meet these characteristics, use was made, as the friction materials, of resin compositions in which potassium titanate fibers, an inorganic filler, an organic filler, and so on are compounded with a thermosetting resin (binder), such as a phenolic resin, for binding the above additives.
Potassium titanate fibers are not carcinogenic, unlike asbestos, and do not damage rotors, unlike metal fibers. However, many types of potassium titanate fibers have an average fiber diameter of 0.1 to 0.5 μm and an average fiber length of 10 to 20 μm, which are not included within a range of fibers recommended by the World Health Organization (WHO) (range of fibers except for WHO fibers as inhalable fibers, i.e., fibrous compounds having an average minor diameter of 3 μm or less, an average fiber length of 5 μm or more, and an aspect ratio of 3 or more). Furthermore, potassium titanate fibers have inadequate wear resistance within a high temperature range.
To cope with this, flaky magnesium potassium titanate, flaky lithium potassium titanate, and amoeboid potassium titanate are proposed in Patent Literatures 1, 2, and 3, respectively.
In addition, the friction materials are required to have excellent fade resistance. A fade phenomenon of a friction material is a phenomenon due to the fact that organic components in the friction material are gasified with increasing temperature of the friction material, resulting in the formation of a gas layer at the frictional interface with a disc. The fade resistance of the friction material can be improved by preventing the formation of the gas layer at the frictional interface. To this end, it is useful to increase the porosity of the friction material to allow easy escape of the gas from the frictional interface. It is conceivable as a method for increasing the porosity of the friction material to adjust and set the forming pressure at a smaller value in the step of binding and forming a source material mixture. However, when the forming pressure is lowered, the friction material decreases its strength and wear resistance and, therefore, cannot achieve good frictional properties.
To cope with this, in Patent Literature 4, hollow powder of an alkaline titanate is proposed which is formed of hollow bodies in each of which alkaline titanate particles having a rodlike, prismoidal, columnar, strip-shaped, granular and/or platy form are bound together.
Meanwhile, friction materials for use in disc brakes, brake linings, and so on of vehicles generally have a high porosity of 10 to 30% for the purposes of increasing the fade resistance and increasing the effectiveness at high speed. Therefore, for example, when a vehicle is parked for a long time or washed, such a friction material is likely to absorb moisture or water. It is known that when water (moisture) adheres to the surface of the friction material or is taken into the friction material, so that the friction material absorbs moisture or water, the coefficient of friction of the friction material becomes extremely high as compared with when the friction material is dry, resulting in the occurrence of brake noise (squeal) during braking.
To cope with this, Patent Literature 5 proposes a friction material containing aramid fibers surface-treated with a silane coupling agent.